1. Field of the Invention
The present invention is directed to mixtures of polyisocyanate adducts based on hexamethylene diisocyanate which have improved compatibility with polyester coreactants and improved solubility in apolar solvents.
2. Description of the Prior Art
The use of polyisocyanates in combination with highly branched polyester polyols prepared from aromatic and/or alicyclic carboxylic acids to form hard coatings which have excellent solvent resistance is known. While it is possible to use both monomeric diisocyanates and polyisocyanate adducts as coreactants for the polyester polyols, the polyisocyanate adducts are preferred, especially in spray applications, due to their lower vapor pressure. Particularly preferred polyisocyanate adducts are the isocyanurate group-containing polyisocyanates based on hexamethylene diisocyanate. In addition to providing better weather resistance than other known polyisocyanate adducts, the isocyanurate group-containing polyisocyanates have low free monomer contents and may be formulated at high solids contents. The low free monomer contents and the ability to formulate these systems using smaller amounts of organic solvents are both important factors from an environmental point of view.
Despite the advantages obtained from using isocyanurate group-containing polyisocyanates, there are compatibility problems between these polyisocyanates and some highly branched polyester polyols used as coreactants. These compatibility problems lead to a hazy appearance in clear coatings produced from these compositions and to pigment flocculation in the corresponding pigmented coating compositions.
Accordingly, it is an object of the present invention to provide a polyisocyanate which maintains the advantages of the known isocyanurate group-containing polyisocyanates, can be formulated to produce clear coatings from unpigmented compositions and does not result in pigment flocculation in pigmented coatings. Surprisingly, these objectives may be achieved from the polyisocyanate mixture set forth hereinafter.